Four Levels of Proof
There are four levels of proof that people are indeed who they say they are. None of them are entirely foolproof, but in order of least to most secure, they are:

1 - What You Know
Passwords are widely used to identify a user, but only verify that somebody knows the password.

2 - What You Have
Digital certificates in the user's computer add more security than a password, and smart cards verify that users have a physical token in their possession, but both laptops and smart cards can be stolen.

3 - What You Are
Biometrics such as fingerprints and iris recognition are more difficult to forge, but you have seen such systems fooled in the movies all the time!

4 - What You Do
Dynamic biometrics such as hand writing a signature and voice recognition are the most secure; however, replay attacks can fool the system.